Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 28, 2017
Valid: June 29-July 3, 2017 (Thursday-Monday)

Summary:
Warm and dry conditions late in the work week will cause concern for an ozone exceedance both Thursday and Friday. Near normal temperatures will return to the NMA as high pressure builds over the SMA. A nearby warm front will bring a few clouds to parts of the western NMA but skies will remain clear for the rest of the region. Slow back trajectories and clear skies will allow ozone to accumulate across the Mid-Atlantic. The focus will be on the I-95 Corridor as the most recent air quality models suggest mid Moderate ozone will develop along the Interstate. As a result, risk for an exceedance will be Appreciable Thursday. Friday is the day of most interest. Southwesterly flow, clear skies and near normal temperatures will promote ozone formation along the I-95 Corridor. The most recent air quality models are in agreement that an isolated USG ozone will occur along the Interstate on Friday. This will bring risk for an exceedance to High. An approaching frontal boundary, and a very humid air mass, will bring clouds to the region both Saturday and Sunday allowing the threat of an air quality exceedance to Slight.

 
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. There continues to be a general agreement with timing and strength of atmospheric features through the end of the medium range period. Weak upper level ridging will build over the Mid-Atlantic by 12Z Thursday, as an upper level trough moves to the Canadian Maritimes. This ridging will be limited both at upper and mid-levels by shortwaves spinning over the Great Lakes. These shortwaves will drive a surface low pressure system that will move across the Great Lakes and into western QC by 12Z Friday. As the surface low passes to the north of the Mid-Atlantic, the associated warm front will lift through parts of northern PA, placing the NMA in the warm sector of this system by 18Z Thursday. By 12Z Friday, most of the Mid-Atlantic will be under quasi-zonal flow aloft as a broad, upper level trough develops over the Midwest. At the same time, shortwaves spinning through the trough will develop another low pressure system over IA/WI. There is some disagreement as to the extent of the eastward progression of the trough but the axis will be roughly over MN/IA/MI by 18Z Friday. The trough will continue to slowly push eastward, developing a center of circulation over eastern MB by 18Z Saturday, encompassing the entire Midwest and western Mid-Atlantic. The low pressure system at the surface will progress over the Great Lakes by 12Z Saturday and into northern ON by 12Z Sunday, pulling a weak cold front into the NMA between 18Z Saturday and 00Z Sunday. This front is expected to stall across the NMA as the upper air support pulls northward, weakening the upper level trough as it moves over the eastern U.S. by 12Z Sunday. The weakening of the trough at upper levels will allow for the westward extension of the Bermuda High to build over the Gulf Coast states, around the trough, and into the Mississippi River Valley, at the same time. A combination of the weakening trough, stalling front and building high pressure to the south will result in high pressure over much of the Mid-Atlantic on Monday with a frontal boundary set up somewhere near the MDL.

The Dailies:

Temperatures will return to near normal for most of the Mid-Atlantic on Thursday. Weak high pressure in the SMA will result in mostly sunny skies in the morning hours, but a nearby warm front in the NMA will bring increasing clouds in the afternoon and evening hours. Cloud cover will be limited to locations west of I-81. Precipitation is unlikely in the NMA as shown in the 03Z SREF, and will be confined to the PA/NY border between 18Z Thursday and 00Z Friday. Clear skies, near average temperatures and southerly/southwesterly flow will be favorable for ozone formation along the I-95 Corridor. This is shown in the 06Z air quality models as a large area of Moderate ozone develops along the interstate, with an isolated area of USG in Maryland on the CMAQ version of the BAMS models. The NOAA and BAMS models keep the widespread Moderate ozone along I-95 and surrounding locations, but the NC air quality model has widespread Moderate ozone along the entire eastern side of the NMA/CMA and the entire SMA. Despite the differences in the models, widespread Moderate ozone is expected for Thursday. Risk for an exceedance will be Appreciable with a focus on the I-95 Corridor.

The threat of an exceedance in the NMA will continue on Friday. Temperatures in the NMA will be above normal as surface flow shifts to southwesterly. Mostly clear skies in the morning will give way to increasing clouds from west to east, but once again locations east of I-81 are expected to remain mostly sunny throughout the day. All of the air quality models are in agreement with a swath of USG ozone developing along the I-95 corridor and an isolated patch in the Susquehanna Valley. With slightly above average temperatures, clear skies, southwesterly flow and strong agreement between the air quality models, it appears likely that an ozone exceedance will occur in the eastern NMA on Friday. In the SMA, skies will remain clear and temperatures will continue to crawl closer to normal, but flow at the surface will shift to onshore. This is appearing in the air quality models as well, as all of the models bring ozone into the Good range despite clear skies and near normal temperatures. As a result of the expected conditions in the NMA, risk for an exceedance will rise to High for Friday.

Unsettled conditions will return to parts of the Mid-Atlantic on Saturday as a cold front approaches from the west. Precipitation ahead of the front is expected to begin between 12Z and 15Z Saturday in western PA, moving eastward to I-81 by 00Z Sunday. Although precipitation is not expected to make it to the I-95 Corridor, surface flow will shift to southerly and onshore for locations in the eastern NMA, reducing ozone accumulation. Onshore flow in the SMA will keep ozone in the Good range despite clear skies through the afternoon, but daytime heating will give way to scattered thunderstorms in the evening hours. As a result of unsettled conditions and onshore flow, risk of an exceedance will lower to Slight for Saturday.

Unsettled conditions will linger across the NMA on Sunday as the cold front stalls in the NMA/CMA. Cloudy skies in the NMA will gradually clear from west to east, giving way to partly cloudy skies by afternoon. Locations in the eastern NMA can expect a few lingering showers in the morning hours but these should dissipate quick enough to not have an impact on air quality for the day. A mostly cloudy skies will persist throughout the day in the SMA, keeping temperatures near normal. Near normal temperatures across the Mid-Atlantic will promote ozone formation, but periods of clouds and a strong southerly flow will limit ozone to the upper Good/Moderate range for Sunday. Risk for an exceedance will remain Slight.

High pressure will begin to build over the Mid-Atlantic on Monday. Mostly sunny skies can be expected across the region, with a few more clouds in the NMA. Temperatures will remain near normal across the region but a shift to westerly flow will drop dew points to below 70 F. Mostly sunny skies, westerly flow, and near normal temperatures will allow for additional ozone formation to take place, increasing the risk for an exceedance to Marginal for Monday.

-Enlow/Ryan